The present invention relates to an optical-fiber communication module, and more specifically, to an optical-fiber communication module that is applied to a Wavelength Division Multiplexing optical-fiber communication system. Furthermore, the present invention relates to applied equipment that uses such optical-fiber communication module.
Because optical-fiber communication provides us with high resistance to electromagnetic noise as well as long-distance, high-speed, and high-capacity communication, the optical-fiber communication enables us to construct a communication system that ensures high reliability. Conventionally, this system transmits light having one wavelength through one optical fiber. However, with the coming of high-capacity information-oriented society in recent years, higher transmission capacity is required. For this reason, technology of Wavelength Division Multiplexing optical-fiber communication system has been put into practical use. According to the technology, a plurality of light having a different wavelength one another are transmitted through one optical fiber, which permits a number of communication channels to be increased for higher capacity. For wavelengths of light transmitted through an optical fiber, a wavelength region having a low transmission loss for the optical fiber is used. A 1.3 μ-region and a 1.5 μ-region are called ‘transmission window’. Because wavelength widths of those windows are limited, narrowing a wavelength interval between channels increases a number of transmission channels. For the present, although a frequency interval is 200 GHz or 100 GHz, the interval tends to become narrower (50 GHz or 25 GHz). If they are expressed in wavelength intervals, they are about 1.6 nm, 0.8 nm, 0.4 nm, and 0.2 nm respectively. If the wavelength interval is narrowed in this manner, it becomes necessary to keep wavelengths of a laser light source constant with accuracy. That is because if one of the wavelengths of the laser light source fluctuates and overlaps with a wavelength of the next channel, a crosstalk with the next wavelength channel arises at a receiving end, which impairs reliable information communication. Those wavelength channels (or frequencies) are called ITU-TS (International Telecommunication Union-Telecommunication Standardization Sector) grid, which is widely known as ITU recommendation. Additionally, wavelength fluctuations of the laser light source due to aging must also be prevented.
Taking such background into consideration, a method for controlling wavelengths of a laser light source used for Wavelength Division Multiplexing optical-fiber communication has been proposed. As an example of this method, there is a method called ‘wavelength locking’ described in Japanese Patent Application Laid-Open No. Hei 11-31859. A basic configuration of this example is shown in FIG. 1. Light emitted from a diode laser 1 is introduced into a fiber 2, and then condensed by a lens 3. The condensed laser light reaches a beam splitter 5 through a first bandpass filter 4. This laser light is reflected by this beam splitter 5, and reaches a bandpass filter 8 that uses a wavelength filter, in other words, a thin film device of dielectric. After that, a light detector PD1 receives transmitted light that has been transmitted through a shoulder slope portion in transmission characteristics of the bandpass filter. On the other hand, a light detector PD2 receives reflected light of the bandpass filter 8. In this example, a photocurrent difference signal between the light detector PD1 and the light detector PD2 is treated as a signal for detecting a wavelength shift. By the way, a reference number 7 is an optical fiber; a reference number 9 is an output-ratio calculating means; and a reference number 10 is a wavelength controlling means.
This example uses a bandpass filter having a single transmission peak. However, only one kind of bandpass filter cannot handle a different wavelength channel. For this reason, in order to handle the ITU-TS grid wavelength channel described above, as many bandpass filters having a different bandpass as a number of channels are required. Producing those many filters results in complicated production control, which is not realistic.
In addition, as clearly shown in FIG. 1, this example has the following mechanism: preparing a wavelength locker module separately from the optical-fiber transmitter module comprising the laser light source; branching a portion of light of the optical-fiber transmitter module and introducing it into the wavelength locker module to detect a wavelength shift; and giving its feedback to the laser light source in the optical-fiber communication module.
In this case, integrating the wavelength locker module into the laser light source module and building the wavelength locker module into the laser light source module enables us to achieve higher cost performance. In addition, regarding to the laser light source module, concurrently preparing monitoring-light for keeping a fiber output constant is required.
Moreover, an example showing that a wavelength-locking portion is built into a laser module is described in Japanese Patent Application Laid-Open No. Hei 10-79723. Additionally, as shown in FIG. 2, because an etalon 18 is used as a wavelength filter, a transmission peak appears repeatedly in response to an order of multiple interference. Because of it, one wavelength filter permits a plurality of wavelength channels to be wavelength-locked. To be more specific, a parallel plane plate (etalon) 18 is located at a tilt in a divergent light path from a laser light source 12. Twin light detectors 20 and 22, which are placed immediately behind the parallel plane plate, split and receive transmitted light. A photocurrent difference between them is a wavelength-shift detecting signal 28. By the way, in the diagram, a reference number 16 is an optical lens; a reference number 12 is a laser emitting source; and a reference number 14 is a diode laser edge. In this case, each of the light that reaches the twin light detectors 20 and 22 must have a different light path length each other when passing through an etalon. In other words, the etalon requires incidence of divergent light or convergent light. That is because: regarding parallel plane waves, there is no light-path difference, resulting in the same signal; therefore, no difference signal is detected between the twin light detectors.
In reality, it is a well-known fact that when an etalon receives incidence of divergent light, finesse (described later) of the etalon decreases effectively. In addition, because the etalon is located at a tilt, a number of multiple reflection at the etalon decreases, causing effective decrease of finesse. The decrease of the effective finesse lowers a resolution of etalon, which causes wavelength locking accuracy to be lowered. Moreover, quantity of divergent light that reaches the twin light detectors 20 and 22 decreases, resulting in lower optical efficiency of laser light. In addition, as regards the laser light source module, concurrently preparing monitoring-light for keeping a fiber output constant is required.